This invention is directed to a class of polypeptides that are useful in modulating the immune response.
Immunoglobulins are proteins produced by plasma cells as one of the latter in a complex sequence of events initiated by antigen challenge of the host. The plasma cells, as another part of this sequence, are derived from B lymphocytes that have become activated by a mechanism or series of mechanisms that, to date, are not fully understood.
There are five classes of recognized immunoglobulins: IgG, IgA, IgM, IgD, and IgE. Of these, IgG represents the major portion of circulating immunoglobulins.
The immunoglobulin protein molecule is composed of four interconnected polypeptide chains, two of which are termed "light" chains and two "heavy" chains. Under this arrangement, the Ig molecule is divided into two identical portions, each of which comprises a light and a heavy chain linked by disulfide bridges formed from cysteine residues. The two resulting portions, in turn, are linked by disulfide bridges.
Each of the four chains is composed generally of two portions, a variable region (V.sub.L and V.sub.H) extending from the amino-terminal and having highly variable amino acid sequences, and a constant region (C.sub.L and C.sub.H) extending from the carboxy terminal and having generally constant amino acid sequences. Each of these regions represents a discrete portion of the Ig molecule. The C.sub.H region is further subdivided into three domains designated C.sub.H 1, C.sub.H 2, and C.sub.H 3 on the basis of constant homology regions.
It has been known for a number of years that the Ig molecule can be enzymatically cleaved into discrete fragments. The properties of these fragments then have been determined using a variety of biological assay systems.
Using papain, the IgG molecule can be cleaved, producing an "Fc" fragment and two "Fab" fragments. The Fc fragment represents the C-terminal portions of the two heavy chains joined by a disulfide bridge, and the Fab fragments are composed of the N-terminal portion of the heavy chain and the entire light chain joined by a disulfide bridge.
Pepsin and plasmin cleavages of the IgG molecule occur at sites closer to the heavy chain C-terminal and downstream of the disulfide bridge that joins the two heavy chains. The pepsin product, termed the "pFc'" fragment, and the plasmin product, thus represent C-terminal portions of the Fc fragment (C.sub.H 3 domain of IgG).
It has been recognized that certain biological activities reside in the Fc and pFc' fragments. Thus, mouse spleen B lymphocytes are induced to proliferate in the presence of papain-derived Fc fragments, [M. A. Berman and W. O. Weigle, J. Exp. Med. 146, 241 (1977)]. Furthermore, it has been observed that the proliferative response is dependent upon the presence of macrophages. It appears that macrophages enzymatically cleave the Fc fragment to a 14,000 MW subfragment, and the latter stimulates B cell proliferation [E. L. Morgan and W. O. Weigle, J. Exp. Med. 150, 256 (1979); and E. L. Morgan and W. O. Weigle, J Exp. Med. 151, 1 (1979)].
Subsequently, it was demonstrated that the Fc fragment has the ability in the presence of both macrophage and T cells to induce a polyclonal antibody response in mouse spleen cells [E. L. Morgan and W. O. Weigle, J. Immun. 124, 1330 (1980)].
Correspondingly, it was also demonstrated that the shorter fragment produced by plasmin digestion of IgG is active in producing a polyclonal antibody response [E. L. Morgan and W. O. Weigle, J. Supra-molecular Structure 14, 201 (1980)].
A class of small peptides now has been discovered. These peptides are useful in modulating the immune response and are available either synthetically using readily available peptide synthesis methods or by enzymatic and chemical cleavage of the IgG molecule. When produced by cleavage methodology, the IgG first is digested with plasmin after which the resulting segments are treated with cyanogen bromide. The resulting active fragment comprises 23 amino acids defined by residues 335-357 of the IgG molecule to which is attached, at the C-terminal, homoserine (Hse) resulting from the methionine (residue 358), the site of the CNBr cleavage. For an illustration of these structures, see, for example the sequence described in G. M. Edelman et al., Proc. Nat'l Acad. Sci. USA 63, 78 (1969).
As noted, the peptides of this invention are available synthetically. When so produced, the C-terminal homoserine may, if desired, be omitted.
It surprisingly has been discovered that relatively small peptides as defined by this invention exhibit a potentiation of the immune response in terms of their ability to activate T cells and natural killer cells, to initiate and/or promote B-cell differentation leading to antibody production, and to regulate an existing immune response. These polypeptides are quite selective in their action in that, unlike the Fc fragment described above, these compounds do not induce a significant B cell proliferation.
It is to a class of such polypeptides that this invention is directed.